PROJECT SUMMARY/ABSTRACT Hydrogen sulfide (H2S) is a novel gaseous mediator that has gained increasing recognition as an important intracellular modulator of cellular signaling, leading to increased anti-inflammatory and cytoprotective responses in models of acute and chronic inflammatory diseases. Paramyxoviruses, in particular, respiratory syncytial virus (RSV), are a major cause of upper and lower respiratory tract infections in children, elderly and immunocompromised hosts, for which no effective treatment or vaccine is currently available. Recent investigations in our laboratory uncovered a critical protective role of H2S in RSV infection, by modulating innate inflammatory responses and viral replication both in vitro and in vivo. Airway epithelial cells infected with RSV display decreased ability to generate H2S and enhanced degradation of H2S, indicating that viral infection leads to changes in H2S cellular homeostasis. Inhibition of H2S generation is associated with enhanced cytokine and chemokine production in vitro, and lack of cystathionine-?-lyase (CSE), one of the best characterized H2S-generating enzymes, results in increased disease severity in a mouse model of infection. Administration of H2S donors result in inhibition of activation of key transcription factors involved in viral- induced proinflammatory gene expression, by affecting their ability to drive gene transcription, but not their nuclear translocation, as well as, in amelioration of clinical disease in mice. In addition to its anti-inflammatory and antiviral activity, H2S administration also reduces viral-induced oxidative cell damage, which we have shown to play an important role in disease pathogenesis, supporting an important modulatory role of the cellular antioxidant pathway in the course of RSV infection. In this application, we will begin to identify the mechanism(s) by which H2S modulates viral-induced cellular signaling and start defining the contribution of H2S-generating enzymes other CSE in cellular antiviral and anti-inflammatory activity. Upon completion of the proposed investigations, we will obtain new critical information regarding the mechanisms of RSV-induced cellular signaling, which may allow us to specifically modulate viral-induced gene expression and therefore, antiviral and innate immune/inflammatory responses. Our results will help elucidate an important and novel molecular pathway by which respiratory viruses modulate lung disease, with strong implications for developing novel therapeutic strategies not only against RSV-induced lower respiratory tract infections (LRTI), but possibly against other important paramyxoviruses, responsible for severe human infections.